Twenty-seven bones turn a forearm into a precision tool
A human hand usually packs twenty-seven bones—fourteen of them phalanges—plus a variable set of sesamoids. Dense nerve endings and unmatched positioning make fingers the body’s richest tactile instruments and partners in gesture, counting, and craft.
Anatomists call the end of a forelimb the manus, but true grasping hands with opposable thumbs belong only to primates—humans, chimpanzees, monkeys, and lemurs. Koalas are a famous near-miss, with two opposable thumbs per forepaw and fingerprints strikingly like ours, and raccoons are said to have hands despite lacking thumbs. Apes and monkeys are sometimes described as four-handed because their big toes oppose like thumbs. An adult man’s hand weighs about a pound.
The skeleton holds 27 bones plus a variable number of sesamoids. Eight carpals sit in two rows at the wrist; five metacarpals form the palm; fourteen phalanges make the digits, three in each finger and two in the thumb. Sesamoid counts differ from person to person: a pair sits at almost every thumb knuckle, while the little-finger knuckle has one in about 82.5% of people and the index finger in about 48%.
The bones form arches that let the hand cup, grip, and flatten. The capitate acts as the rigid keystone of the lower carpal arch, and the middle-finger metacarpal is the stiffest ray, while the thumb’s metacarpal meets only the trapezium and moves independently. Thumb, index, and middle finger together manage most delicate grips; the ring and little fingers wait in reserve for power grips. Long flexors and extensors have their muscle bellies in the forearm, and the two thumb extensor tendons outline the hollow known as the anatomical snuffbox.
Fingertips carry some of the body’s densest nerve endings. Each hand is controlled mainly by the opposite brain hemisphere, so handedness reflects how an individual brain is organized. Ten digits, and the twelve finger bones a thumb can touch, have seeded counting systems, and hands carry whole languages in sign.
Source: Hand
